The present invention relates to novel isocyanate-terminated prepolymers having a viscosity of less than about 6000 mPa.cndot.s at 80.degree. C. and an NCO content of about 3 to 10%. This invention also relates to a process for making these novel prepolymers, a process of producing polyurethane/polyurea elastomers from these novel prepolymers, and the elastomers produced therefrom.
Polyurethane casting compositions are described in U.S. Pat. No. 3,211,701. These comprise the reaction product of an isocyanate-terminated interpolymer with an organic crosslinking agent (which may be a polyamine or a polyhydric alcohol). The isocyanate-terminated interpolymer is prepared from a hydroxyl-terminated polyester urethane formed by reacting a polyester having a molecular weight of 900 to 1200 with an organic diisocyanate, with an excess of organic diisocyanate.
Isocyanate-terminated prepolymers are broadly described in U.S. Pat. No. 3,963,681. These prepolymers may be cured with an aromatic or aliphatic polyamine or polyol. Suitable isocyanates for the prepolymers include aromatic or aliphatic diisocyanates and triisocyanates. The polyurethane/urea elastomers of the '681 patent are based on mixtures of Polymegs of two different molecular weights (one high and one low, with ca. 1,000 to 4,500 average MW of the blend). These blends of polyols are described as resulting in better high temperature and dynamic properties than a single Polymeg having the same average MW as the above blend.
Prepolymers and polyurethane/ureas produced therefrom are described in U.S. Pat. Nos. 3,766,148 and 3,600,358. The prepolymers in both patents are based on methylene bis(4-phenylisocyanate). The '148 patent describes chain extending these methylene bis(4-phenylisocyanate) based prepolymers with MDA (4,4'-methylenedianiline) to form polyurethane/ureas. U.S. Pat. No. 3,600,358 describes prepolymers based on methylene bis(4-phenylisocyanate) with neopentyl adipate (or other esters of neopentyl glycol). These prepolymers are subsequently chain extended with aromatic diamines, preferably MDA, to form polyurethane/urea elastomers.
U.S. Pat. No. 3,115,481 also describes polyurethane/urea elastomers. These elastomers are formed by first preparing foams and subsequently crushing the cells in a heated press. This invention uses prepolymers wherein an aromatic isocyanate is present on the prepolymer ends. This increases the reactivity of the isocyanate prepolymers such that they are suitable for reaction with water to form foams. However, this high reactivity makes this type of prepolymer too fast to process with diamine chain extenders to form cast elastomers.
Polyurethane/urea elastomers are also described in U.S. Pat. No. 3,789,032. These elastomers are based on aliphatic isocyanate terminated prepolymers. The elastomers produced in this reference are also produced by reacting the aromatic isocyanate terminated prepolymers with water. Increased reactivity is also a problem in these prepolymers.
U.S. Pat. No. 4,098,773 describes prepolymers that are prepared from linear polyols and aliphatic isocyanates, cycloaliphatic isocyanates, aliphatic-aromatic isocyanates, sterically hindered aromatic isocyanates or 4,4'-methylene bis(phenylisocyanate) where the OH:NCO ratio is 1:1.1 to 1:2. They subsequently add a symmetrical aromatic diisocyanate to the prepolymer and chain extend with symmetric diols to prepare polyurethanes with high softening points.
U.S. Pat. No. 3,997,514 describes isocyanate terminated prepolymers prepared from mixtures of aromatic and aliphatic diisocyanates. These prepolymers comprise the reaction product of excess polyesters or polyether polyols with an aromatic diisocyanate to form a hydroxyl terminated prepolymer. This OH terminated prepolymer subsequently reacts with an excess of aliphatic diisocyanate to form a diisocyanate terminated prepolymer.
These prepolymers of U.S. Pat. No. 3,997,514 can be prepared in either a one-step or two-step process. In the two-step process, the aromatic diisocyanate must be mixed with a stoichiometric excess of glycol. More specifically, the glycol to aromatic diisocyanate mole ratio must be about 2:1. The intermediate OH terminated prepolymer is allowed to react with the excess aliphatic diisocyanate in a second reaction step. In the one-step process, all of the components are combined in a single reaction step. The intermediate OH terminated polyurethane prepolymer from the aromatic isocyanate and the polyol is generated in situ, and reacts further with the aliphatic diisocyanate to form an aliphatic isocyanate terminated polyurethane prepolymer. Polyurethane/polyurea elastomers can be produced by curing these prepolymers with aromatic diamines.
The prepolymers of the '514 patent have the disadvantage that many of them have high viscosities and therefore must be processed at high temperature (&gt;100.degree. C.) to allow molds to be filled with the viscous mixtures of prepolymers and chain extenders. This is problematic for processing as it requires that the prepolymers be stored at temperatures of 100.degree. C. or higher. High temperature storage limits the storage lifetime of the prepolymers and results in increased energy costs for the manufacturer.
Most of the examples of the '514 patent use toluene diisocyanate as the aromatic isocyanate. This particular isocyanate (TDI) has a high vapor pressure and thus is less desirable from an industrial hygiene viewpoint.
In addition, MDA is used as a chain extender in preparing the elastomers described by the '514 reference. At the time of the invention, MDA was considered less hazardous than MOCA, which was already recognized as a carcinogen. Since that time, MDA has also been identified as a suspect human carcinogen. The potential for exposure to the MDA is considerable due to the high temperatures used when MDA is used as a chain extender for these polyurethane prepolymers. Simply replacing the MDA with a mixture of isomers of diethyltoluene diamine in the production of elastomers in the '514 reference is not possible. This substitution results in the mixture reacting too quickly to allow casting in open molds.
The '514 patent addressed the viscosity problem of the prepolymers by addition of a plasticizer which reduced the viscosity (See Example 10 of the '514 patent). Plasticizers, however, have the disadvantage that they slowly migrate from the finished molded article during its use. This phenomenon is well known and can occur by volatilization of the low molecular weight plasticizer or movement of it to the surface of the part. As the plasticizer is removed, the properties (hardness, flexibility, etc.) of the polymer change.
In addition, the plasticizer itself can be problematic. For example, plasticizers are known to be responsible for fogging in automobile windshields. Furthermore, potential exposure of people to the plasticizer can lead to additional hygiene concerns depending on the polymers end use application.
It has been surprisingly found that one can make a relatively low viscosity isocyanate-terminated prepolymer in one-step by reacting a polyether polyol with a mixture of aliphatic diisocyanate and aromatic diisocyanate in which the polyether polyol to aromatic diisocyanate mole ratio is significantly lower than 2:1. It has also been surprisingly found that such prepolymers can be mixed with aromatic diamine extenders at 80.degree. C. and cast into a mold with sufficient pot life to be practical for production. These prepolymers react similarly with aromatic diamines as do prepolymers prepared from only aliphatic diisocyanates.